311 research outputs found
Suppression of intrinsic neutron background in the Multi-Grid detector
One of the key requirements for neutron scattering instruments is the
Signal-to-Background ratio (SBR). This is as well a design driving requirement
for many instruments at the European Spallation Source (ESS), which aspires to
be the brightest neutron source of the world. The SBR can be effectively
improved with background reduction. The Multi-Grid, a large-area thermal
neutron detector with a solid boron carbide converter, is a novel solution for
chopper spectrometers. This detector will be installed for the three
prospective chopper spectrometers at the ESS. As the Multi-Grid detector is a
large area detector with a complex structure, its intrinsic background and its
suppression via advanced shielding design should be investigated in its
complexity, as it cannot be naively calculated. The intrinsic scattered neutron
background and its effect on the SBR is determined via a detailed Monte Carlo
simulation for the Multi-Grid detector module, designed for the CSPEC
instrument at the ESS. The impact of the detector vessel and the neutron
entrance window on scattering is determined, revealing the importance of an
optimised internal detector shielding. The background-reducing capacity of
common shielding geometries, like side-shielding and end-shielding is
determined by using perfect absorber as shielding material, and common
shielding materials, like BC and Cd are also tested. On the basis of the
comparison of the effectiveness of the different shielding topologies and
materials, recommendations are given for a combined shielding of the Multi-Grid
detector module, optimised for increased SBR.Comment: 26 pages, 18 figures, revise
Efficient Color-Dressed Calculation of Virtual Corrections
With the advent of generalized unitarity and parametric integration
techniques, the construction of a generic Next-to-Leading Order Monte Carlo
becomes feasible. Such a generator will entail the treatment of QCD color in
the amplitudes. We extend the concept of color dressing to one-loop amplitudes,
resulting in the formulation of an explicit algorithmic solution for the
calculation of arbitrary scattering processes at Next-to-Leading order. The
resulting algorithm is of exponential complexity, that is the numerical
evaluation time of the virtual corrections grows by a constant multiplicative
factor as the number of external partons is increased. To study the properties
of the method, we calculate the virtual corrections to -gluon scattering.Comment: 48 pages, 23 figure
Helac-Phegas: a generator for all parton level processes
The updated version of the Helac-Phegas event generator is presented. The
matrix elements are calculated through Dyson-Schwinger recursive equations.
Helac-Phegas generates parton-level events with all necessary information, in
the most recent Les Houches Accord format, for the study of any process within
the Standard Model in hadron and lepton colliders
Feynman Rules for the Rational Part of the QCD 1-loop amplitudes
We compute the complete set of Feynman Rules producing the Rational Terms of
kind R_2 needed to perform any QCD 1-loop calculation. We also explicitly check
that in order to account for the entire R_2 contribution, even in case of
processes with more than four external legs, only up to four-point vertices are
needed. Our results are expressed both in the 't Hooft Veltman regularization
scheme and in the Four Dimensional Helicity scheme, using explicit color
configurations as well as the color connection language.Comment: 18 pages, 11 figures. Misprints corrected in Appendix A. Version to
be published in JHE
Color-flow decomposition of QCD amplitudes
We introduce a new color decomposition for multi-parton amplitudes in QCD,
free of fundamental-representation matrices and structure constants. This
decomposition has a physical interpretation in terms of the flow of color,
which makes it ideal for merging with shower Monte-Carlo programs. The
color-flow decomposition allows for very efficient evaluation of amplitudes
with many quarks and gluons, many times faster than the standard color
decomposition based on fundamental-representation matrices. This will increase
the speed of event generators for multi-jet processes, which are the principal
backgrounds to signals of new physics at colliders.Comment: 23 pages, 11 figures, version to appear on Phys. Rev.
A simulational study of the indirect geometry neutron spectrometer, BIFROST at the European Spallation Source, from neutron source position to detector position
The European Spallation Source (ESS) is intended to become the most powerful
spallation neutron source in the world and the flagship of neutron science in
the upcoming decades. The exceptionally high neutron flux will provide unique
opportunities for scientific experiments, but also set high requirements for
the detectors. One of the most challenging aspects is the rate capability and
in particular the peak instantaneous rate capability, i.e. the number of
neutrons hitting the detector per channel or cm at the peak of the neutron
pulse. The primary purpose of this paper is to estimate the incident rates that
are anticipated for the BIFROST instrument planned for ESS, and also to
demonstrate the use of powerful simulation tools for the correct interpretation
of neutron transport in crystalline materials. A full simulation model of the
instrument from source to detector position, implemented with the use of
multiple simulation software packages is presented. For a single detector tube
instantaneous incident rates with a maximum of 1.7 GHz for a Bragg peak from a
single crystal, and 0.3 MHz for a vanadium sample are found. This paper also
includes the first application of a new pyrolytic graphite model, and a
comparison of different simulation tools to highlight their strengths and
weaknesses.Comment: 45 pages, 20 figure
ee4fgamma, A program for e+e- -> 4f, 4fgamma with nonzero fermion masses
A computer program ee4fgamma for calculating cross sections of any four
fermion final state of e+e--annihilation at high energy and the corresponding
bremsstrahlung reaction that is possible in the framework of the Standard Model
is presented. As the fermion masses are arbitrary, the cross sections can be
computed without any collinear cut, the on-shell top quark production can be
studied and the Higgs boson exchange can be incorporated in a consistent way.
The program can be used as a Monte Carlo generator of unweighted events as
well.Comment: 20 pages; version published in Computer Physics Communication
Reducing full one-loop amplitudes to scalar integrals at the integrand level
We show how to extract the coefficients of the 4-, 3-, 2- and 1-point
one-loop scalar integrals from the full one-loop amplitude of arbitrary
scattering processes. In a similar fashion, also the rational terms can be
derived. Basically no information on the analytical structure of the amplitude
is required, making our method appealing for an efficient numerical
implementationComment: updated versio
Tensorial Reconstruction at the Integrand Level
We present a new approach to the reduction of one-loop amplitudes obtained by
reconstructing the tensorial expression of the scattering amplitudes. The
reconstruction is performed at the integrand level by means of a sampling in
the integration momentum. There are several interesting applications of this
novel method within existing techniques for the reduction of one-loop multi-leg
amplitudes: to deal with numerically unstable points, such as in the vicinity
of a vanishing Gram determinant; to allow for a sampling of the numerator
function based on real values of the integration momentum; to optimize the
numerical reduction in the case of long expressions for the numerator
functions.Comment: 20 pages, 2 figure
Super AutoDipole
The publicly available package for an automated dipole subtraction,
AutoDipole, is extended to include the SUSY dipoles in the MSSM. All fields in
the SM and the MSSM are available. The code is checked against the analytical
expressions for a simple process. The extended package makes it possible to
compute the QCD NLO corrections to SUSY multi-parton processes like the stop
pair production plus jets at the LHC.Comment: 16 pages, 1 figure, v2: a few typos to match the published version in
Eur. Phys. J.
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